Device miniaturization by reducing circuit loss is requested for a power conversion device that operates in a wide range of input/output voltages. Power conversion devices typically include semiconductor elements, and a magnetic component such as a transformer or a reactor, and mainly switching loss of the semiconductor elements, copper loss and iron loss of the magnetic component depend on drive frequency of the power conversion device. Varying the drive frequency in response to operating conditions of the power conversion device is conceived as a method of reducing loss of the power conversion device.
For example, there is proposed, for a power conversion device including an inverter configured with a plurality of semiconductor elements and an isolation transformer, a method of selecting on the basis of a detected outputs a frequency that minimizes loss of the power conversion device from among prepared table, data indicating a relationship between the outputs and the drive frequency (see Patent Document 1).
There is further proposed a method of driving a DC-DC converter that includes a full-bridge inverter, an isolation transformer, and a rectifier circuit. In the method, the drive frequency for the DC-DC converter is varied throughout from a lower limit to an upper limit within an operable range of the DC-DC converter every time the operating condition changes, to build in each time a relationship between the whole loss or the power conversion efficiency of the DC-DC converter and the drive frequency; and the curve indicating the relationship is searched for two or more local extremal points, to drive the DC-DC converter at a frequency that brings about the minimum loss or the maximum power conversion efficiency among frequencies of the two or more local extremal points (see Patent Document 2).